a) Field of the Invention
The present invention relates to an apparatus and a method for measuring an ambient physical parameter, and for example high hydrostatic pressure, applied to a highly birefringent sensing optical fiber.
b) Description of the Prior Art
Ambient physical parameters such as temperature, pressure and magnetic fields affect the propagation properties of optical fibers. Birefringent sensing fibers are a special type of optical fibers. They are characterized by two orthogonal polarization states allowing simultaneous propagation of two orthogonal polarization modes. Upon application of an ambient physical parameter, the two orthogonal polarization modes propagate at different velocities, thus creating a relative delay between the two orthogonal polarization modes, proportional to a value of the ambient physical parameter. For example, to measure pressure, U.S. Pat. No. 4,920,261 (Bock et al.) describes the use of a birefringent fiber placed in a pressure chamber, connected to a light source and a measuring apparatus. To measure the relative delay between the two orthogonal polarization modes introduced by pressure, the measuring apparatus detects light signals in a predetermined direction of polarization, locks-in the detected light signals, measures the amplitude of the light signals according to time, and uses a computer to determine the relative delay between the two orthogonal polarization modes using the amplitude of the light signals according to time. This apparatus requires many calculations to determine the ambient pressure. Moreover, this apparatus implies a trade-off between sensitivity and range of measurements since the length of the sensing fiber determines the range of the measurements while the precision of the measurements is indirectly proportional to the range of measurements.